Generally, as an external recording medium for data backup of a computer and the like, there exists a magnetic tap cartridge composed by housing a single reel in winch a magnetic tape of a recording medium is wound (see FIG. 2 in Japan patent laid open publication 11-265560).
There exist various kinds of cartridges as this kind of magnetic tape cartridge, and in recent years, for the purpose of downsizing a drive unit, a magnetic tape cartridge which is composed so as to form a pulling-out port of a magnetic tape at a corner of side wall of a cartridge case and to open/close the pulling-out port by a curved shape door formed like a laterally long sheet has been developed to make the space of movements of a tape pulling-out mechanism small in the drive unit.
In such the magnetic tape cartridge, in order to open/close the door, structure is contemplated in that: making the door a universally slidable structure and providing the door with a door-side spring hooking portion; providing an inner surface of the cartridge case with a case-side spring hooking portion; always energizing the door toward its closing direction by a tensile coil spring hooked across the door-side spring hooking portion and case-side spring hooking portion; and pushing/moving a protrusion piece for operation formed on an outer surface of the door by an operating arm appended to a loading mechanism, thereby opening the door.
Then, as a method to automate building-in work of the door and tensile coil spring in the cartridge case in the manufacturing process of magnetic tape cartridge of such the structure, a building-in method which firstly grips the door with a robot hand and sets it in a guide groove, then grips the tensile coil spring with the robot hand, and attaches its one end to the door-side spring hooking portion provided in the door and its other end to the case-side spring hooking portion provided in an inner surface of the cartridge case is contemplated.
However, in the building-in method, if the door fitted in the guide groove oscillates and/or slants left/right, a position of the door-side spring hooking portion for the cartridge case displaces, so there exists a problem that work of attaching both ends of the tensile coil spring to the door-side spring hooking portion and the case-side spring hooking portion becomes difficult. Moreover, if a gap between the door and case-side spring hooking portion is narrow, there exists a problem that, in the work of attaching both ends of the tensile coil spring to the door-side spring hooking portion and the case-side spring hooking portion, it becomes difficult to insert the robot hand gripping the tensile coil spring between the door and case-side spring hooking portion. That is, in the building-in method, there exists a problem that the building-in work of the door and tensile coil spring is difficult.